Fuel pump relay socket difference in voltages -2007 5.4 3V Expy

[Mike Expy Poland]

Hi everyone,

Thanks for stopping and reading. Any chance I could hear your thoughts and suggestions what and how to check, based on your experience with voltages / voltage drops / fuel pump?

Fuel pump is working. No faults found (I could not check live data thought). Recently I began however getting misfires on 3rd cylinder.

I am betting on spark plugs / coils, as my car has also a custom liquid gas system, quite new, high quality, and it's recording same misfires, not related to fuel supply (i.e. fuel supply in the liquid gas system is all good, so I'm blaming ignition).

I am a bit perplexed however and I felt, before seeing misfires on 3rd cylinder, that I am having a bit less power under foot than before. So perhaps I am having two issues at once,the second being fuel pump operation / efficiency, since:

my fuel pump relay socket has strange voltages, at least not what I would expect.. I quickly drew it below. I have +12, +11.6 (voltage drop of 0,4V), +0,17V and zero. Shouldn't I have (with relay disconnected and only after the two seconds for fuel pump to be primed) two +12 Vs and two 0Vs?

Does it mean it's time to dig under fuse box in engine compartment?

Many thanks for any thoughts on this!

 

[Anthony DeVor]

Hi everyone,

Thanks for stopping and reading. Any chance I could hear your thoughts and suggestions what and how to check, based on your experience with voltages / voltage drops / fuel pump?

Fuel pump is working. No faults found (I could not check live data thought). Recently I began however getting misfires on 3rd cylinder.

I am betting on spark plugs / coils, as my car has also a custom liquid gas system, quite new, high quality, and it's recording same misfires, not related to fuel supply (i.e. fuel supply in the liquid gas system is all good, so I'm blaming ignition).

I am a bit perplexed however and I felt, before seeing misfires on 3rd cylinder, that I am having a bit less power under foot than before. So perhaps I am having two issues at once,the second being fuel pump operation / efficiency, since:

my fuel pump relay socket has strange voltages, at least not what I would expect.. I quickly drew it below. I have +12, +11.6 (voltage drop of 0,4V), +0,17V and zero. Shouldn't I have (with relay disconnected and only after the two seconds for fuel pump to be primed) two +12 Vs and two 0Vs?

Does it mean it's time to dig under fuse box in engine compartment?

Many thanks for any thoughts on this!

View attachment 40515

You are correct - pin 85 or 86 should have 12v+ and the other should be ground (0v)

Pin 30 (or 87 - however they have it wired from the factory) should have 12v+ and the other in theory shouldn’t be anything (if it doesn’t sense ground through the fuel pump) or 0v (if it does sense ground)

It sounds like you have something back feeding somehow.

ALTHOUGH. As you said you should only have 12v+ on pin 85 or 86 for a couple seconds with the fuel prime timing then it should go away and then you should only have 12v+ on pin 30 (or 87)


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[JamaicaJoe]

You need to provide more information. A schematic of the relay perhaps.

A single pole relay with only 4 terminals implies that one coil terminal and the armature will be directly connected to 12V. The second coil terminal would normally be pulled to zero to run the pump and I suggest that is the 11.6 V terminal that is floating from a pull up resistor in the ECU because the ECU is not calling for fuel. The zero volt terminal probably goes to the fuel pump but since the relay is out, it is naturally zero V. I could be wrong from the limited information.

EDIT I misread the 0.17 as 12V . Yes you should have two terminals pulled to 12V one from ECU that might be zero, or 0.17, and the last with zero because it is the pump and you have the relay out.

The .4 V voltage drop could be a concern. Any corrosion?

Show a picture of the schematic in the manual with the voltages measured on the appropriate terminals.

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